Introduction

The family Arcturidae Dana, 1849 groups filter feeding marine isopods which are globally widespread from shallow waters to the deep sea. This family includes 14 genera (Poore and Schotte 2015): Agularcturus Kensley, 1984, Amesopous Stebbing, 1905, Arctopsis Barnard, 1920, Arcturina Koehler, 1911, Arcturinella Poisson & Maury, 1931, Arcturinoides Kensley, 1977, Arcturopsis Koehler, 1911, Arcturus Latreille, 1829, Astacilla Cordiner, 1793, Edwinjoycea Menzies & Kruczynski, 1983, Idarcturus Barnard, 1914, Neastacilla Tattersall, 1921, Parastacilla Hale, 1924, Spectrarcturus Schultz, 1981. Only three of them, Arcturina, Arcturinella, and Astacilla, have been found in the Mediterranean Sea.

The genus Arcturinella, and its single species A. banyulensis, were described by Poisson and Maury (1931) from material collected in the Bay of Banyuls-sur-Mer, located on the French coast (north-western Mediterranean). The type species for this genus and species was reported on a single female collected in 1927.

Arcturinella differs from Arcturina by presenting body not geniculate, the posterior margin of P4 not excavated, and the pleotelson with all segments coalescent; and from Astacilla because the body is not geniculate between pereonites 4 and 5, and pereopod 4 is reduced.

Materials and methods

The study area (40°38′57.3”–40°38′53.8”N/0°52′02.7”–0°51′59.1”W) was located in the inner continental shelf area at 4.71 miles offshore from the Ebro Delta river mouth, 2.04 miles offshore the coast and at depths between 11 and 14 m (Fig. 1). The estuarine continental shelf of the Ebro Delta river mouth is characterized by its shallow depths, salinities between 37.5 and 38.2 psu, and water temperatures ranging between 18 and 22 °C (Bastida et al. 2012). Sampling was part of the DELTA campaigns performed in years 2013 and 2014, which aimed to monitor the community structure of macrofauna (among others) along a period of 3 years and to relate it to their environment. Different types of samples were taken, e.g., macrofauna, benthic fish (Solea solea, S. senegalensis and Mullus ssp.), and sediments. Macrofauna sampling was carried out using a Van Veen grab (KC DENMARK A/S) of 0.05 m2 on sandy bottom. Grab samples were carefully sieved over a mesh size of 0.5 mm and stored in >90% ethanol. Samples were examined under a Leica EC3 stereomicroscope with camera, and digital images were captured by the LAS-EZ V2 software (Leica Microsystems). Specimens were found at the same location area during 2 consecutive years [May 2013 (DELTA 0) and April 2014 (DELTA 1)]. All specimens have been lodged in the collections of the Museo Nacional de Ciencias Naturales, Madrid (MNCN).

Fig. 1
figure 1

Distribution of the species of Arcturinella. a Position of the three locations in which the two species have been recorded. b Detail of Ebro Delta, including the localization of the current sampling station. (■ A. banyulensis, ● A. deltensis)

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Results

Systematics

Family Arcturidae Dana,1849

Genus Arcturinella Poisson & Maury, 1931

Diagnosis

Body flattened dorsoventrally, not geniculate between pereonites 4 and 5. Cephalon fused with pereonite 1, incomplete suture at the margins. Pereonite 4 hexagonal and widest, with posterior margin not excavated. Pleotelson without free segments, with lateral wings. Mandibles rounded, pars incisiva and pars molaris absent. Pereopods 1–4 reduced; pereopods 1–3 flattened and with articles not fused; pereopod 1 unguis absent; pereopods 2–4 dactyli absent; pereopod 4 vestigial, linear, with circular or angular section, with or without fused articles. Brood-pouch of oostegites of pereonites 2–4. Pereopods 5–7 with secondary unguis smaller than the primary unguis.

Arcturinella deltensis sp. nov. (Figs. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12)

A. banyulensis Rodríguez-Sánchez et al., 2001

Holotype

Ovigerous female, total body length 6.0 mm, pereonite 4 length 1.5 mm and width 3.5 mm; some pleopods damaged; sampling campaign DELTA 0, St. 2 (40° 38’53.8”N – 0°51’59.1”E, 14.4 m), Ebro Delta continental shelf, Van Veen grab, 9 May 2013; habitus [MNCN 20.04/10601] and 19 slides [MNCN 20.04/10602a to MNCN 20.04/10602s]

Paratype

Ovigerous female, total body length 5.8 mm, pereonite 4 length 1.4 mm and width 3.2 mm; some pleopods damaged; same sampling campaign; habitus [MNCN 20.04/10603]

Other material

Two females (one of them, ovigerous); total body lengths 6.0 mm (ovigerous); habitus [MNCN 20.04/10604]; and 5.7 mm (non-ovigerous, with antennae and some pereopods 5–7 damaged); habitus [MNCN 20.04/10605] and 5 slides [MNCN 20.04/10606a to 20.04/10606e]; sampling campaign DELTA 1, St. 3 (40° 39’36.4”N – 0°51’38.0”E, 11 m), Ebro Delta continental shelf, Van Veen grab, 8 April 2014. One female; habitus [MNCN 20.04/5571]; sampling campaign FAUNA I (MNCN), off Estepona (between 36° 18.25’and 36° 20.20’ N, between 05° 13.25’and 05° 12.40’ W), bottom trawl, 60–62 m, 16 July 1989.

Type locality

Off Ebro Delta, St. 2 (40° 38’53.8”N – 0°51’59.1”E, 14.4 m).

Etymology

After the type locality. Deltensis means from a delta. Although there are several deltas in Catalonia, when referring to “delta” the Ebro Delta is understood, which is by far the most important due to its characteristics and greater extension compared to the rest.

Description of the holotype

Body up to 6.0 mm, excluding antennae, entirely depressed, with pereonite 4 longest, more than twice as wide as long (Figs. 2 and 3). Integument smooth, transparent, showing almost always a sort of double line along the margin. Head almost square, with eyes in soft bulges and dorsal pair of broad, low protuberances. Pereon surface not sculpted. Pleotelson without any free segment and with lateral incision at slightly less than one-third of length; lateral wings at slightly less than two-thirds of length.

Fig. 2
figure 2

Arcturinella deltensis sp. nov.. a–e Photographs from the specimens sampled. a Holotype ovigerous female [MNCN 20.04/10601], habitus in dorsal view. b Holotype ovigerous female [MNCN 20.04/10601], habitus in lateral view. c Holotype ovigerous female [MNCN 20.04/10601], cephalon and pereon in ventrolateral view. d Holotype ovigerous female [MNCN 20.04/10601], pleotelson in ventral view, showing the uropods. e Paratype ovigerous female [MNCN 20.04/10603], habitus in dorsal view. Scale bars: a–c, e: 1 mm; d: 0.5 mm

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Fig. 3
figure 3

Arcturinella deltensis sp. nov.. Holotype ovigerous female [MNCN 20.04/10601]. a Habitus in dorsal view. b Habitus in lateral view. c Cephalon and pereon in ventrolateral view (pereopods 5 omitted). d Pleotelson in ventrolateral view (pereopods 6–7 omitted). Scale bar: a–d: 1 mm

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Antenna 1 (Fig. 4a) basal articles with angular section, covered with short setae; flagellum with two apical aesthetascs, equal in size, one plumose seta and a group of three setae near to apex. Antenna 2 (Fig. 4b, c) 30% of body length; ratio of peduncular articles 2–5 and flagellum are 1.0:1.3:2.2:2.2:1.4; robust and short setae on the peduncle articles, especially near on anterior margin; tubercles absent; flagellum three-articulate, with distal claw. Mandibles (Fig. 5a, b) almost symmetrical, with rounded inner and inferior margins, covered by spine-like setae, longer and thin on medial margins; incisor, lacinia mobilis and molar undistinguishable. Maxilla 1 (Fig. 5c) inner lobe with three terminal stout plumose setae; a group of four simple setae near the apex; outer lobe with nine stout tooth-like apical setae forming two rows; rows of fine setae along inner and outer margins. Maxilla 2 (Fig. 5d) inner lobe with eight plumose robust setae, and six thinner plumose setae on apical margin; middle lobe with three stout plumose setae; outer lobe with two stout long plumose setae. Maxilliped (Fig. 6a, b) with endite bearing fine simple setae on both sides and superior margin; 5–6 plumose setae on distal and mesial margin; two coupling hooks; palp with numerous plumose setae on mesial margin and apex; outer margin with few long plumose setae; relative lengths of articles 1.0:1.4:2.2:1.4:1.2.

Fig. 4
figure 4

Arcturinella deltensis sp. nov.. Holotype ovigerous female [MNCN 20.04/10601]. a Antenna 1. b Antenna 2. c A2, detail of the apex. Scale bars: a, c: 0.1 mm; b: 1 mm

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Fig. 5
figure 5

Arcturinella deltensis sp. nov.. Holotype ovigerous female [MNCN 20.04/10601]. a Left mandible. b Right mandible. c Maxilla 1. d Maxilla 2 (ventral view). Scale bar: a–d: 0.1 mm

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Fig. 6
figure 6

Arcturinella deltensis sp. nov.. Holotype ovigerous female [MNCN 20.04/10601]. a Right maxilliped (ventral view). b Endite of left maxilliped (dorsal view). c Uropod. d U, detail of apex. Scale bars: a, b, d: 0.1 mm; c: 1 mm

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Pereopod 1 (Fig. 7a, b) with numerous plumose and denticulate setae on posterior margin of merus–propodus; mesial face of propodus setose; dactylus 40% of length of propodus, with five setae, one of them at apex; unguis absent. Pereopods 2–3 (Figs. 7c, d and 8a, b) flattened, with angular section; articles of pereopods 2–3 conspicuous; long plumose setae on their posterior margins; numerous robust and short setae on merus–propodus of pereopods 2–3; ratio of lengths of articles from basis to propodus 4.2:1.5:2.1:1.2:1.0 (pereopod 2) and 8.0:2.0:2.6:1.4:1.0 (pereopod 3); dactyli P2-3 absent. Pereopod 4 (Figs. 8c, d and 9a) thin and with angular section; basis, ischium, merus and carpus fused in a single piece, with a sort of membrane closing over the surface of oostegite; propodus short, with rounded apex; dactylus absent; apical long plumose seta at apex of right P4, not observed in the other counterpart; robust and short setae on the basis–carpus and propodus. Three pairs of oostegites (Pereopods 2–4); oostegites 2–3 thin and similar; oostegite 4 thickened compared with anterior ones. Oostegites 2–4 oval, with length:width ratios 1.2, 1.3 and 1.2, respectively, surrounded by fine short setae. Pereopods 5–7 (Figs. 9b-e and 10) long and similar, slightly decreasing in size; presence of a bulb on anterior margin of carpus; dactylus with secondary unguis on posterodistal angle, and two groups of two and three setae on posterior and anterior side respectively; unguis half of dactylus length.

Fig. 7
figure 7

Arcturinella deltensis sp. nov.. Holotype ovigerous female [MNCN 20.04/10601]. a Pereopod 1. b P1, detail of apex. c Pereopod 2 with oostegite (ventral view). d P2, detail of apex. Scale bars: a: 0.5 mm, b, d: 0.1 mm; c: 1 mm

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Fig. 8
figure 8

Arcturinella deltensis sp. nov.. Holotype ovigerous female [MNCN 20.04/10601]. a Pereopod 3 with oostegite (ventral view). b P3, detail of apex. c Right pereopod 4 with oostegite (ventral view). d P4, detail of apex. Scale bars: a, c: 1 mm; b, d: 0.1 mm

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Fig. 9
figure 9

Arcturinella deltensis sp. nov.. Holotype ovigerous female [MNCN 20.04/10601]. a Left pereopod 4, detail of apical articles showing the sort of membrane closing over the surface of oostegite. b Pereopod 5. c P5, detail of carpal bulb. d P5, detail of unguis. e P5, detail of secondary unguis (setae omitted). Scale bars: a, c, d, e: 0.1 mm; b: 1 mm

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Fig. 10
figure 10

Arcturinella deltensis sp. nov.. Holotype ovigerous female [MNCN 20.04/10601]. a Pereopod 6. b P6, detail of carpal bulb. c P6, detail of unguis. d P6, detail of secondary unguis (setae omitted). e Pereopod 7. f P7, detail of carpal bulb. g P7, detail of unguis. h P7, detail of secondary unguis (setae omitted). Scale bars: a, e: 1 mm; b, c, d, f, g, h: 0.1 mm

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Pleopod 1 (Fig. 11a) with retinaculum of four coupling hooks; endopod and exopod with two lateral and seven apical plumose setae each; in addition, exopod with one lateral plumose setae at level of one half of length of outer margin. Pleopod 2 (Fig. 11b) slightly shorter and wider than pleopod 1; endopod with eight apical plumose setae; exopod with 12 apical plumose setae. Pleopods 3–5 (Fig. 12) similar; with two, one, and one plumose setae on external margin of exopod, respectively. Uropodal exopod less than half length of endopod, with two distal plumose setae (Fig. 6c, d).Pale brown variegated color in alcohol.

Fig. 11
figure 11

Arcturinella deltensis sp. nov.. Female [MNCN 20.04/10605]. a Pleopod 1. b Pleopod 2. Scale bar a–b: 0.1 mm

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Fig. 12
figure 12

Arcturinella deltensis sp. nov.. Female [MNCN 20.04/10605]. a Pleopod 3. b Pleopod 4. c Pleopod 5. Scale bar a–c: 0.1 mm

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Variation

Habitus thinner in the ovigerous female paratype [MNCN 20.04/10603] (Fig. 2e). The female from Estepona [MNCN 20.04/5571] shows the dorsum of pereonites 5–7 with the joints between the segments and with the pleotelson more conspicuous; pleotelson of Estepona individual less inflated, with the apical half more acute than in the type specimens. Female from Estepona [MNCN 20.04/5571] of whitish color.

Habitat and distribution

Soft sedimentary bottoms of the western Mediterranean sea at depths 11–14 m.

Remarks

Some pleopods damaged in holotype and paratype. For this reason the pleopods of the non-ovigerous female 5.7 mm [MNCN 20.04/10605] were used in the description. After inquiring to the MNHN of Paris, the Observatoire Océanologique–Laboratoire Arago from Banyuls-sur-Mer, and the Monaco Museum without success, the holotype of A. banyulensis is currently missing. Poisson and Maury (1931) pointed out that the figure of habitus is made with a semi-schematic style. This has difficulty the identification of the species, it is not known to what extent the figure is real. Furthermore, some details (e.g., most mouthparts and pleopods) were not described by Poisson and Maury (1931) and cannot no longer be checked. Certainly there are differences in shape and proportions, in the cephalon and pleotelson and especially in pereopod 4. It seems difficult to believe that Poisson and Maury (1931) distinguished articulations between articles if they did not actually exist. This feature by itself justifies the creation of a new species. Table 1 summarizes the main differences between the two known species of Arcturinella (A. banyulensis vs. A. deltensis); note that Poisson and Maury (1931) misinterpreted the articles of P2-4 because dactylus is absent.

Table 1 Comparison of Arcturinella species using data provided by Poisson and Maury (1931) and the present study
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Discussion

Taxonomy

The specimens were initially identified as Arcturinella banyulensis, and the first objective was to perform a new description of the species; in the absence of the holotype, it would be designated a neotype. However, because of the various reasons given in the remarks and Table 1, A. banyulensis and A. deltensis sp. nov. are considered different species. Accounting for these clear differences between A. deltensis and A. banyulensis, the authors believe it is more prudent to describe the new species rather than taking for granted that Poisson & Maury (1931) misinterpreted the specimen and incorrectly described A. banyulensis. Particularly, without knowing that they were truly wrong (without evidence) and without being absolutely certain that A. banyulensis still exists. Only if ever Banyuls-sur-Mer area is deeply sampled and A. deltensis is repetitively found, would anyone venture (but without having the certainty) to synonymize A. deltensis and A. banyulensis or simply to give A. banyulensis as a “lost or ghost species” (which could also be) and follow forward with A. deltensis. In this case, the solution would be to synonymize both species, but at least a complete description of the species would remain.

The males of Arcturinella are still unknown. Members of the family Arcturidae generally have a significant sexual dimorphism; e.g., some Arcturopsis males present a medial odd sexual appendix used to hold the female during copulation. Mediterranean Astacilla specimens present also a noteworthy sexual dimorphism (males are usually longer and more slender than females and do not have dorsal tubercles); however, usually males do not present specific morphological characters, such as dorsal tubercles, allowing the separation among the distinct species. Although it is still unknown, some of these sexual dimorphisms character may be also found in this species. Unfortunately, neither the authors that described A. banyulensis nor ourselves after the examination of all specimens of A. deltensis found any male within this genus.

Biogeography

The family Arcturidae contains 151 species, from which 13 (included in the three genera indicated above) are recorded in the Mediterranean Sea. Of these, some species are commonly recorded, such as Astacilla longicornis (Sowerby, 1806) and Astacilla mediterranea Koehler, 1911 which have a widespread Mediterranean distribution. However, the majority of species are restricted to the western Mediterranean area, as in the case of Arcturinella deltensis sp. nov.. The habitat of this species is the fine sandy bottom beaches. The type locality and the presence of the species also in the Strait of Gibraltar area confirms its presence on the Western Mediterranean coast, and may help to predict its presence near deltas or estuaries of the east coast of the Iberian Peninsula, as Guerao and Castelló (2012) also pointed out for Parachiridotea mediterranea Roman, 1991 (Isopoda, Valvifera, Chaetilidae). The soft sediments are rather unsampled; probably a more intensive sampling will end up with wider geographical ranges, and hopefully the male of this species will be described in the near future.

Ecology

It is evident that the two species represent a case of sympatric speciation. Probably the speciation process may be due to adaptation to different substrates; unfortunately, there is no clear data on the substrate in which the specimen of A. banyulensis was obtained by dredging along Cape Béar, although Poisson and Maury (1931) indicated that the specimen was kept alive in a crystallizer for several days and shifting on seaweed. In the case of A. deltensis, the sampled bottom was sedimentary typical of deltas or estuaries, without algae, contrary to what can be deduced from the case of A. banyulensis. A. deltensis sp. nov. has mandibles different from other arcturids. Although arcturids are considered filter feeders, they present the typical mandible with incisor and molar, which gives them ability to grind food. By contrast, the new species has undifferentiated mandibular parts and many spine-like setae; this may really be interpreted as a typical filter mechanism. Conversely, in the new species P1-3 have few setae and consequently they have diminished their filtration capacity; their morphology suggests that they are not filtering anymore, and so P4, which is much reduced. These could be morphological adaptations to a sedimentary feeding type typical from these bottoms.